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A new skill of modulating driving signals of a synthetic jet with low frequency is intro-duced to enhance the control authority. Based on dynamic mesh technique, the effect of flow con-trol over a NACA0015 airfoil with a Synthetic Jet Actuator (SJA) has been investigated. The SJA is located at x/c=10％, and two jet injection angles of 90° and 30° have been considered. Flow struc-tures indicate that modulation with low frequency can reduce the dissipation rate of vortices after they shed off the surface. Thus aerodynamic performance of the airfoil could be improved. For the injection angle of 90°, modulation can increase the control authority of the SJA. Compared to the unmodulated case, increment of lift-to-drag ratio is up to 15％. The modulating frequency corre-sponding to the largest increment is F+M≈1. For the injection angle of 30°, improvement of aero-dynamic forces positively relates to momentum coefficient of the SJA. Due to the decrease of effective momentum, modulation degrades the effect of flow control. The opposite responses to modulation are believed to have a relationship with the variation of primary control mechanism. When jet injection angle varies from 90° to 30°, the primary mechanism changes from the interac-tion of vortices and waves to the direct momentum addition. The skill of modulation of driving sig-nals is very useful for applications of SJA.